bpo-41316: Make tarfile follow specs for FNAME (GH-21511)

tarfile writes full path to FNAME field of GZIP format instead of just basename if user specified absolute path. Some archive viewers may process file incorrectly. Also it creates security issue because anyone can know structure of directories on system and know username or other personal information.

RFC1952 says about FNAME:
This is the original name of the file being compressed, with any directory components removed.

So tarfile must remove directory names from FNAME and write only basename of file.

Automerge-Triggered-By: @jaraco
(cherry picked from commit 22748a8)

Co-authored-by: Artem Bulgakov <ArtemSBulgakov@ya.ru>

Lib/tarfile.py

@@ -420,6 +420,8 @@ def _init_write_gz(self):
         self.__write(b"\037\213\010\010" + timestamp + b"\002\377")
         if self.name.endswith(".gz"):
             self.name = self.name[:-3]
+        # Honor "directory components removed" from RFC1952
+        self.name = os.path.basename(self.name)
         # RFC1952 says we must use ISO-8859-1 for the FNAME field.
         self.__write(self.name.encode("iso-8859-1", "replace") + NUL)
 

Lib/test/test_tarfile.py

@@ -1416,12 +1416,15 @@ def write(self, data):
                                    pax_headers={'non': 'empty'})
             self.assertFalse(f.closed)
 
+
 class GzipWriteTest(GzipTest, WriteTest):
     pass
 
+
 class Bz2WriteTest(Bz2Test, WriteTest):
     pass
 
+
 class LzmaWriteTest(LzmaTest, WriteTest):
     pass
 
@@ -1464,8 +1467,17 @@ def test_file_mode(self):
         finally:
             os.umask(original_umask)
 
+
 class GzipStreamWriteTest(GzipTest, StreamWriteTest):
-    pass
+    def test_source_directory_not_leaked(self):
+        """
+        Ensure the source directory is not included in the tar header
+        per bpo-41316.
+        """
+        tarfile.open(tmpname, self.mode).close()
+        payload = pathlib.Path(tmpname).read_text(encoding='latin-1')
+        assert os.path.dirname(tmpname) not in payload
+
 
 class Bz2StreamWriteTest(Bz2Test, StreamWriteTest):
     decompressor = bz2.BZ2Decompressor if bz2 else None

Misc/ACKS

@@ -243,6 +243,7 @@ Colm Buckley
 Erik de Bueger
 Jan-Hein Bührman
 Lars Buitinck
+Artem Bulgakov
 Dick Bulterman
 Bill Bumgarner
 Jimmy Burgett

Misc/NEWS.d/next/Library/2020-07-28-12-08-58.bpo-41316.bSCbK4.rst

@@ -0,0 +1 @@
+Fix the :mod:`tarfile` module to write only basename of TAR file to GZIP compression header.